The present invention relates to a motor, in particular a motor that has a bearing wear monitoring device. The monitoring device monitors the wear of the bearings that support the rotor, caused by the displacement of the rotor.
Canned motors, also referred to as sealed motors, are used in plants that require high reliability. The bearings of a canned motor wear from use and must be monitored to prevent damage to the motor. It is essential to have a bearing wear monitoring device to monitor the wear of the bearings that support the rotor. A conventional bearing wear monitoring device of the prior art is shown in FIGS. 13 and 14.
The conventional bearing wear monitoring device has a pair of detection coils 103a and 103b as shown in FIGS. 13 and 14. The coils 103a and 103b are attached to the iron core teeth of a stator 102. The coils run along the entire circumference of the teeth in the longitudinal direction (the axial direction). The coils are separated apart by 180 degrees and are connected to produce a differential output voltage which is read and monitored by a voltmeter 104.
A rotation of the rotor 105 causes a voltage to be induced in the detection coils 103a and 103b. The higher-harmonic voltage due to rotor-groove 105a is superimposed on the fundamental-harmonic voltage that is synchronized with the frequency of the power source. Since the outputs of detection coils 103a and 103b are separated by 180 degrees and are interconnected to produce a differential output, the fundamental-harmonic voltage is canceled out at the voltmeter 104. Thus the voltmeter 104 detects and displays only the difference of the instantaneous values of the higher-harmonic voltages. When a bearing in a motor becomes worn, the gaps d1 and d2 between stator 102 and rotor 105 are caused to change. Therefore, the voltage displayed on the voltmeter 104, due to the higher-harmonic voltage of detection coils 103a and 103b, corresponds to the changes in gaps d1 and d2 which is the effect of a worn bearing.
The bearing wear monitoring device of the prior art can only monitor (detect) the bearing wear in the radial direction that is caused by radial (axial) displacement of rotor 105. The prior art device cannot monitor the bearing wear in the axial direction (thrust direction). In a canned motor of this type, the load of rotor 105 on the rotor shaft and the direction of wear on the bearing vary with the nature and pressure of the fluid that is being transported. Thus, there is a need to detect non-directional bearing wear, a requirement which this bearing wear monitoring device cannot meet.
To address the drawbacks of the prior art, the applicant filed Japanese patent application 08-236483, on Sep. 6, 1996, for a canned motor equipped with the type of bearing wear monitoring device shown in FIGS. 10-12. As shown in FIG. 10, the bearing wear monitoring device in this canned motor has a total of eight detection coils C1-C8 provided in pairs positioned 180 degrees apart on both ends of the axial (longitudinal) direction of stator 52.
As shown in FIG. 11, a detection circuit 54 is provided for detecting wear of the bearing in the axial direction. Facing pairs of detection coils C2, C4 and C6, C8, from FIG. 10, are connected in series. The series connected coils are wired to a filter 59. The filter 59 provides a filtered signal output to the amplifier 58. Amplifier 58 provides gain to the measured signal from the coils through the filter to drive the indicator 53 with a differential signal. The differential signal connected to the indicator 53 corresponds to the wear of the bearing in the axial direction.
Referring now to FIGS. 12, a detection circuit 56 is provided for detecting wear of the bearing in the radial direction. Opposing pairs of series connected detection coils C1, C3 and C5, C7 are series connected to diodes 55. Each set of series connected diode 55 and series connected coils are connected together in parallel providing a differential output signal. Amplifier 58 provides gain to the differential output signal and drives indicator 53 with the gained differential signal. The gained differential signal connected to the indicator 53 corresponds to the wear of the bearing in the radial direction.
The conventional wear monitoring device shown in FIGS. 10-12 monitors wear of the bearing in both the axial and radial direction. The axial direction of the bearing is detected by detection circuit 54, and wear in the radial direction, including the oblique direction, is detected by detection circuit 56. Thus it is possible to monitor the state of wear of the bearing non directionally.
The above described bearing wear detection device can detect wear of the bearing both radially and axially, and can monitor bearing wear with good precision. However, it has been found through subsequent experimentation that the detection device has great difficulty in adjusting the zero point in the axial direction. The zero point is adjusted by positioning the stator 52 and a rotor not shown in the axial direction.
For example, if the bearing becomes worn due to use of the canned motor, either the rotor or stator 52, or both, must be replaced. However, when the rotor or stator 52 of an existing canned motor is replaced, it is in practice impossible to control the dimensions between the part that is replaced and the part that is not replaced. Unless stator 52 and the rotor are manufactured as a pair, their widths assembled into the canned motor cannot be controlled as was done when they were first manufactured.
When a part is replaced, care must be taken to ensure that the parts are within the prescribed dimensional displacement in the axial direction with respect to the non-replaced part. If one of the two parts shifts with respect to the other, then the voltage of the detection coils will change significantly. For Example, a replacement-part the rotor shifts in either axial direction in an attempt to adjust this displacement mechanically. This in turn causes the output voltage of detection coils C2 and C4 and detection coils C6 and C8 on both axial ends of stator 52 to change.
It is a very difficult operation to achieve agreement between the mechanical positioning (zero-point adjustment) of the rotor and stator 52 and the electrical zero-point adjustment of the bearing wear monitoring device. In practice the zero point of the bearing wear monitoring device is matched to the mechanical zero point by means of an electrical process such as adjustment of the gain of the detection circuits 54 and 56. The operation of adjusting the zero point of the bearing wear monitoring device by positioning stator 52 and the rotor is very involved and difficult. This zero point adjustment is a problem not only when stator 52 or the rotor 57 is replaced or repaired, but also when stator 52 and the rotor are assembled in the manufacturing stage.